According to Shannon's sampling theorem, the sampling frequency should be twice the frequency being sampled. The nyquist frequency is the maximum frequency that can be sampled without aliasing. In ultrasound imaging, it is defined as half of the pulse repetition frequency.
NF = PRF/2 (nyquist frequency = pulse repetition frequency/2)
This is the so-called Nyquist limit. If the velocity of flow exceeds the Nyquist limit, the direction and velocity are inaccurately displayed and appear to change direction. Color flow Doppler capitalizes on this effect. This allows detecting flow disturbances from laminar to turbulent flow.

See also Aliasing Artifact, Repetition Rate, and Sampling Rate.

(PWD) Pulsed wave (PW) Doppler is a Doppler ultrasound mode that evaluates blood flow velocities in a range specific area along the length of the sound beam. Measured are changes in received frequency due to relative motion (flow) between a sound source (transducer) and sound receiver (transducer).
PW Doppler produces an audible signal as well as a graphical representation of flow. The Doppler shift produced by moving blood flow is calculated by the ultrasound system.

See also Amplitude Indicator, Pulsed Ultrasound.

A flowmeter derives an accurate measure of the transit time ; transit time (or time of flight) is the difference in time for an ultrasonic pulse to travel with the direction of flow and against it. With bi-directional illumination it is the time taken for an ultrasound pulse to travel from one transducer to another. This time is used to measure blood flow rate. The difference between the upstream and downstream measured transit times calculates the volume flow.

See also Echo Ranging.

Variance is the modification of Doppler frequencies within each pixel during a pulse packet with color Doppler flow imaging. Variance is mapped to the color green and is used to detect turbulence. At low variance (laminar flow) pixels within each range gate are encoded in red or blue according to flow direction. At high variance (turbulent flow) a percentage of pixels within each range gate are encoded with green according to the degree of spectral broadening.

Color assignments can be switched on any ultrasound system and are used as directional indicators. The colors of red and blue indicates flow direction in color Doppler.
Red is encoded to represent flows toward the transducer.
Blue pixel encoding is used to depict flows away from the transducer.

See also Directional Color Power Doppler.